Rifles are problematic as a single category since that involves hunting rifles. We can look at machine guns as a category that is neither rifle or handgun, and with limited use in hunting.
That said, the death rate of machine gun ownership may still be lower than handgun ownership, with a possible explanation being that it is easier to buy a handgun compared to a machine gun. A further jump into the data would then explore who are the buyer demographic of a handgun compared to a machine gun, and how machine guns compare to handguns in locations where both has similar restrictions on those who buy them.
An other avenue to explore would be to look at purchases of ammunition to determine which guns are actually in use for non-murder purposes. Hunting ammunition for example may show a much more common use than guns used for non-hunting purposes, and thus have a lower murder rate per shot. Machine guns and handguns might share a much more similar profile in that regard, possible normalized towards how much ammunition each use in a target range.